In the boring of a workpiece, the latter is generally provided with a roughed-out hole or bore which must be turned internally to the desired diameter with the desired tolerance by a boring tool carried at an end of a shaft which extends into the hole of the workpiece and is rotated as part of a spindle unit while the workpiece is maintained stationary. The tool, which may be an oxide-ceramic in the case of finish boring of holes to high-precision tolerances, projects laterally from the ends of the spindle shaft which is journaled in a housing capable of being advanced to shift the tool further into the workpiece.
While fine boring is generally carried out with a relatively limited cutting force, the tool, the workpiece, the spindle unit and the chuck arrangement elastically deform during the process. Upon retraction of the tool from the finished bore, the cutting force is relieved and the elastically deformed parts relax which can cause the tool to spring against the finished surface of the bore and cut grooves in the latter. Even when these grooves are of slight depth, they detrimentally affect the surface finish, the useful life of the workpiece when the latter is employed as a bearing, and even the service life of the part formed with the bore and the tool. For this reason, tool-liftoff devices have been proposed to prevent the tip of the tool from engaging the finished surface during withdrawal of the boring tool.
In the brochure "Das Gleitstutzenlager in hydrodynamischen FAG-Spindeln," publication No. 44 110 Kugelfischer Georg Schafer & Co., Werk Ebern, Germany, at page 11, there is described and illustrated a fine-boring spindle provided with an effective tool-liftoff device. When the working end of the spindle unit reaches its final position and machining is to be concluded, the spindle unit is braked. An annular piston built into the shaft end is hydraulically displaced somewhat to impart play to the bearings. A further annular piston behind the bearing at the working end of the arrangement is then displaced so that its conical bore bears against a pin projecting from the shaft in the plane of the tool to displace the latter away from the surface of the workpiece and thereby shift the center or axis of the shaft for retraction relative to the working axis of the shaft.
While this arrangement has been found to be highly suitable for many purposes, it has been found to be disadvantageous in the respect that the spindle unit must be brought to standstill before liftoff is effected. The stopping of the spindle arrangement with on and off switching devices is, naturally, time consuming and and may be detrimental when the shaft is journaled in hydrodynamic sleeve bearings. In this case, each time the machine is started or stopped, a transition between the mixed friction and solid-body friction characteristics must occur. As is well known with hydrodynamic bearings of this type, such transitions are detrimental to the life of the unit provided with the bearing.
In German printed application (Auslegeschrift) DT-AS 24 09 721, there is described a liftoff device in which the liftoff of the tool edge from the finished surface of the workpiece is effected by radial shifting of the shaft. In this case, use is made of a circumferential liquid-pressure cushion. A disadvantage of this arrangement is the requirement that the force transmission (hydraulic medium or toggle lever) the effective through the rotating shaft which causes problems such as sealing, friction losses and the like. The problem is especially pronounced and the system less convenient when it is necessary to provide a hollow shaft to facilitate transfer of the tool.